Disk spring contacts, brush contacts, and clip contacts have been mainly used for electric contacts, such as connectors, switches and terminals. Frequently used composite materials for the contacts comprise a relatively inexpensive substrate, such as a copper alloy and stainless steel, having excellent corrosion resistance and mechanical properties, and the substrate is coated with silver, which is excellent in electrical characteristics and solderability.
Among the composite materials for contacts described above, those using stainless steel for the substrate are able to make contacts of small size, since they are superior in mechanical characteristics and fatigue life compared with composite materials for contacts using a copper alloy. Accordingly, they are used for movable contacts, such as a tactile push switch and a sensing switch, that are required to have long life. The materials are frequently used for push buttons for mobile phones in recent years, in which the action frequency of the switches is rapidly increasing due to diversification of mailing functions and Internet functions.
However, while stainless steel coated with silver is able to make a switch small in size while increasing the action frequency, compared with copper alloy coated with silver, there has been a problem that the life is shortened due to wear of the silver, since the pressure at the contacts in the switch is large.
As a stainless steel strip coated with silver or a silver alloy, ones in which a substrate is plated with nickel, are frequently used. However, silver at the contacts is peeled off due to wear with an increased action frequency of the switch, when such a stainless steel strip is used for the switch. As a result, the nickel plating layer of the substrate is exposed to the air, which increases contact resistance, and failures ascribed to mal-continuity become evident. In particular, this phenomenon is liable to occur in dome-shaped movable contacts having a small diameter, which has been a crucial technical problem for further miniaturization of the switch.
To solve the problem, palladium is plated on the nickel plating layer, with additional gold plating thereon. However, electrical resistance increases at the contacts, since palladium is inferior in conductivity.
Therefore, nickel, copper, nickel, and gold are sequentially plated on stainless steel, to improve electrical conductivity. However, cracks appear at the upper layer during bending due to the hardness of nickel plating, to deteriorate corrosion resistance by making the underlying layer expose to the air, although nickel plating itself is excellent in corrosion resistance.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.